Open Outlook and go to the File menu.
Select Import/Export.
In the Import and Export Wizard, select Export to a file and click Next.
Select Outlook Data File (.pst) and click Next.
Select the OST file that you want to convert and click Next.
Choose whether to export a specific folder or the whole account. Check the Include subfolders box if you want to export all the subfolders of the selected folder.
Click Next and select a location for the PST file.
Click Finish to start the conversion process.

Greetings,
Peter